Many methods of treating spinal disorders are known in the art. One known method involves anchoring a screw or a hook to the vertebrae, and fixing the screws or hooks along a spinal rod to position or immobilize the vertebrae with respect to one another. The screws or hooks commonly have heads with U-shaped channels in which the spinal rod is inserted and subsequently clamped by a set screw or other fastener or locking cap. This method may commonly involve multiple screws or hooks, as well as multiple spinal rods. With this method, the spinal rod(s) may be shaped to maintain the vertebrae in such an orientation as to correct the spinal disorder at hand (e.g., to straighten a spine having abnormal curvature). Additionally or alternatively, the screws or hooks may be spaced along the rods(s) to compress or distract adjacent vertebrae.
Surgeons have often encountered considerable difficulty with this method because of trouble aligning the spinal rod(s) with the U-shaped channels in the heads of the screws or hooks. For example, the heads of the screws or hooks are often out of alignment with one another because of the curvature of the spine or the size and shape of each vertebrae. To facilitate easier insertion of the spinal rods into the U-shaped channels, and to provide additional flexibility in the positioning of the spinal rods and the screws and hooks, screws and hooks have been developed wherein the head (which includes the U-shaped channel) initially pivots with respect to the anchor member (e.g., screw shank or hook). One example of such a screw is disclosed in U.S. Pat. No. 5,586,984 to Errico et al., which is incorporated herein by reference.
The process of positioning and setting known bone anchors may be tedious and relatively time consuming, typically requiring more than one surgical tool to clamp the spinal rod and anchor member in desired positions. Even with a high degree of skill and care, the process of positioning a known set screw or other fastener on the bone anchor and then manipulating the screw or fastener to clamp or re-clamp the spinal rod and anchor member in place can take more time than desired during a surgical procedure or even resulting in the rod, anchor member, or both moving out of position before clamping is completed.
Therefore, a need still exists for bone anchors including polyaxial bone anchors that provide an improved mechanism for clamping the spinal rod and anchor member in their desired positions.